Background: Pediatric obesity is a multifactorial disease characterized by an imbalance between energy intake and expenditure. In rare cases, it is caused by underlying medical disorders arising from disruptions in e.g. the leptin-melanocortin pathway, which regulates satiety and energy expenditure.

Aim: To investigate resting energy expenditure (REE) and body composition in children and adolescents with severe obesity with or without underlying medical causes.

Methods: This prospective observational study included pediatric patients who underwent an extensive diagnostic workup in our academic centre, assessing genetic, hypothalamic, endocrinologic and medication-induced causes of obesity, REE by indirect calorimetry and body composition by air displacement plethysmography. The ratio between measured REE (mREE) and predicted REE (pREE, Schofield formula) was expressed as REE%, with decreased/elevated mREE defined as REE% ≤90%/≥110%. Additionally, the ratio between mREE and fat-free-mass (FFM) was calculated.

Results: We included 289 patients, of which 65 (23%) with an underlying medical cause: non-syndromic/syndromic genetic obesity (n= 30 [10%]/27 [9%], respectively), hypothalamic obesity (n= 9 [3%]), and medication-induced obesity (n= 6 [2%]). Across all patients, mean age was 10.7 ± 4.4 years, 60% were female, mean BMI SDS was 3.7 ± 1.1, indicating severe obesity. Mean REE% was 100.5% ± 13.1; 58 (20%) patients had a decreased vs. 67 (23%) an elevated mREE. Mean mREE/FFM was 46.5 ± 10.6 kcal/kg FFM/day and did not differ between patients with vs. without underlying medical causes (p-values >0.05).

Conclusions: In this cohort of children with severe obesity due to various etiologies, large interindividual differences in mREE were found. Substantial minorities of 20% and 23% of patients had decreased or elevated mREE, respectively. This knowledge is important for patient-tailored treatment, e.g. personalized dietary and physical activity plans as well as consideration of pharmacologic treatment affecting central energy expenditure in children with decreased mREE.